1. Field of the Invention
This invention relates to a radiation image storage panel for use in radiation image recording and reproducing techniques, in which phosphors are utilized.
2. Description of the Related Art
Radiation image recording and reproducing systems, in which radiation image storage panels (i.e., stimulable phosphor sheets) comprising a substrate and a layer of a stimulable phosphor overlaid on the substrate are utilized, have widely been used in practice.
With the radiation image recording and reproducing systems, for example, radiation carrying image information of an object, such as a human body, is irradiated to the radiation image storage panel, and a radiation image of the object is recorded on the radiation image storage panel. Thereafter, the radiation image storage panel, on which the radiation image of the object has been stored, is exposed to stimulating rays, which cause the radiation image storage panel to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the radiation image storage panel, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal, which represents the radiation image, by photoelectric read-out means.
The image signal, which has been obtained from the radiation image recording and reproducing systems described above, is then subjected to image processing, such as gradation processing and processing in the frequency domain, such that a visible radiation image, which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness, is capable of being obtained. The image signal having been obtained from the image processing is utilized for reproducing a visible image for diagnosis, or the like, on film or on a cathode ray tube (CRT) display device. In cases where the radiation image storage panel, from which the image signal has been detected, is then exposed to erasing light, and energy remaining on the radiation image storage panel is thereby released, the erased radiation image storage panel is capable of being used again for the recording of a radiation image.
Novel radiation image read-out apparatuses for use in the radiation image recording and reproducing systems described above have been proposed in, for example, Japanese Unexamined Patent Publication Nos. 60(1985)-111568, 60(1985)-236354, and 1(1989)-101540. In the proposed radiation image read-out apparatuses, a line light source for irradiating linear stimulating rays onto a radiation image storage panel is utilized as a stimulating ray source, and a line sensor comprising a plurality of photoelectric conversion devices arrayed along the length direction of a linear area of the radiation image storage panel, onto which linear area the stimulating rays are irradiated by the line light source, is utilized as photoelectric read-out means. (The length direction of the linear area of the radiation image storage panel is referred to also as the main scanning direction.) Also, the proposed radiation image read-out apparatuses comprise scanning means for moving the radiation image storage panel with respect to the line light source and the line sensor and in a direction, which is approximately normal to the length direction of the linear area of the stimulable phosphor sheet. (The direction, which is approximately normal to the length direction of the linear area of the stimulable phosphor sheet, is referred to also as the sub-scanning direction.) With the proposed radiation image read-out apparatuses, wherein the linear stimulation of the radiation image storage panel and the linear readout from the radiation image storage panel are performed by the utilization of the line light source and the line sensor, the emitted light detection time is capable of being kept short, the size of the apparatus is capable of being kept small, and the cost is capable of being kept low.
In the radiation image read-out apparatuses utilizing the line sensors, the light is emitted in a scattered form from the radiation image storage panel. Therefore, a light guide member and a light collecting mirror are located in the vicinity of the position, which is exposed to the stimulating rays, and the light emitted in the scattered form from the radiation image storage panel is reflected by the light collecting mirror toward the light guide member. In this manner, the efficiency, with which the light emitted by the radiation image storage panel is collected, is enhanced. Alternatively, a distributed index lens array, such as a SELFOC lens array or a rod lens array, is utilized in order to enhance the degree of collection of the light, which has been emitted from each of exposed sites of the radiation image storage panel, on the line sensor.
In radiation image read-out apparatuses, when the radiation image is read out from the radiation image storage panel, the radiation image storage panel is set, for example, in a horizontal state or a vertical state. Therefore, it often occurs, depending upon the state in which the radiation image storage panel is set for the image read-out operation, that the radiation image storage panel is deflected by gravity, though not buckled, and the horizontality of the radiation image storage panel becomes low under the influence of gravity.
In particular, in the cases of small-sized radiation image read-out apparatuses, the image readout from the radiation image storage panel is ordinarily performed from the side opposite to a radiation irradiation side. In such cases, such that the stimulating rays irradiated to the radiation image storage panel and the light emitted by the radiation image storage panel may not be blocked, supporting of the radiation image storage panel is performed only from two sides extending in the longitudinal direction of the radiation image storage panel (i.e., the sub-scanning direction). Therefore, in such cases, the horizontality of the radiation image storage panel is apt to become low.
If the horizontality of the radiation image storage panel becomes low, the beam size of the stimulating rays impinging upon the surface of the radiation image storage panel will vary, and the efficiency, with which the light emitted by the radiation image storage panel is collected, will become low. As a result, the image quality of the image reproduced from the image signal detected from the radiation image storage panel will become bad. Particularly, in the cases of the small-sized radiation image read-out apparatuses described above, an optical lens is ordinarily utilized for collecting the light, which has been emitted by the radiation image storage panel. However, the allowance range of the optical lens with respect to the lowering of the horizontality of the radiation image storage panel is narrow. Therefore, the tendency of the collected light quantity to decrease due to, for example, a shift of a point, upon which the stimulating rays impinge, is reinforced, and the image quality of the image reproduced from the image signal detected from the radiation image storage panel is caused to become bad.
As a technique for enhancing the horizontality of the radiation image storage panel, it may be considered to set the thickness of a glass plate or a carbon plate acting as the substrate of the radiation image storage panel at a large value. However, with such a technique for enhancing the horizontality of the radiation image storage panel, the radiation image storage panel itself becomes heavy and inconvenient for processing. Also, in cases where the irradiation of the radiation is performed from the substrate side of the radiation image storage panel, the problems will occur in that absorption of the radiation by the substrate occurs markedly, and the image quality of the image reproduced from the image signal detected from the radiation image storage panel becomes bad.